#include <iostream>

#include <string>

using namespace std;

class String

{

public:

    String(const char* str = NULL);//通用构造函数,String("abc")

    String(const String &str);//拷贝构造

    ~String();

    String& operator=(const String &str);//赋值运算符。返回引用

    String operator+(const String &str) const;

    String& operator+=(const String &str);//+=操作符。返回引用

    char& operator[](int n) const;//下标操作符。返回引用

    bool operator==(const String &str) const;

    int size() const;//字符串实际大小，不包括结束符

    const char *c_str() const;//将string转为char *

private:

    char *data;

    int length;

};

String::String(const char* str)//通用构造

{

    if (!str)

    {//为空。String a()

        length = ;

        data = new char[];

        *data = '\0';

    }

    else

    {

        length = strlen(str);

        data = new char[length + ];

        strcpy(data, str);//会拷贝源的结束符

    }

}

String::String(const String &str)//拷贝构造，深拷贝

{

    length = str.size();

    data = new char[length + ];

    strcpy(data, str.c_str());

}

String::~String()

{

    delete[] data;

    length = ;

}

String& String::operator=(const String &str)//赋值操作符4步

{

    if (this == &str) return *this;//1 自我赋值，返回自身引用

    delete[] data;//2 删除原有数据

    length = str.size();//3 深拷贝

    data = new char[length + ];

    strcpy(data, str.c_str());

    return *this;//4 返回自身引用

}

String String::operator+(const String &str) const//+操作符3步

{//新建对象包括新空间，拷贝两个数据，返回新空间

    String newString;

    newString.length = length + str.size();

    newString.data = new char[newString.length + ];

    strcpy(newString.data, data);

    strcat(newString.data, str.data);

    return newString;

}

String& String::operator+=(const String &str)//+=操作符5步

{//重分配新空间，拷贝两个数据，删除自己原空间，赋值为新空间，返回引用

    length += str.size();//成员length是实际长度

    char *newdata = new char[length + ];

    strcpy(newdata, data);

    strcat(newdata, str.c_str());

    delete[] data;

    data = newdata;

    return *this;

}

char& String::operator[](int n) const

{//下标操作符，返回引用

    if (n >= length) return data[length - ];//如果越界，返回最后一个字符

    else return data[n];

}

bool String::operator==(const String &str) const

{

    if (length != str.size()) return false;

    return strcmp(data, str.c_str()) ? false : true;

}

int String::size() const

{

    return length;

}

const char *String::c_str() const

{

    return data;

}

int main()

{

    char a[] = "Hello", b[] = "World!";

    String s1(a), s2(b);

    cout << s1.c_str() << endl;

    cout << s2.c_str() << endl;

    s1 += s2;

    cout << s1.c_str() << endl;

    s1 = s2;

    cout << s1.c_str() << endl;

    cout << (s1 + s2).c_str() << endl;

    cout << s1.size() << endl;

    cout << s1[] << endl;

    if (s1 == s2)

        cout << "相等" << endl;

}